1. Field of the Invention
The invention is concerned with movable vane assemblies of the type used in gas turbine engines or compressors for interaction with a gas stream. In such assemblies a plurality of radially aligned vanes are circumferentially arranged within an annular passage formed by a housing. The vanes are supported for rotation about their radial axis to vary the effective cross-sectional area of the annular passage. A plurality of rotatable gear means are connected for rotation with the aligned vanes and a ring gear is provided having teeth means adapted for simultaneous engagement with the teeth means of the rotatable gear means such that rotation of the ring gear causes rotation of the vanes. More particularly the invention is concerned with improvements in such an assembly.
2. Prior Art
The prior art discloses variable geometry turbine nozzles in which vanes are cantilevered from shafts which rotate in bushings supported by an outer housing of an engine. Actuation generally proceeds via a spur gear sector which is attached to an outer end of each of the shafts of the vanes. The gears and vanes are driven by a face gear which is often made integral with the outer shaft of a large diameter ball bearing. The inner race of this ball bearing is generally mounted to the engine case. The vanes are located radially by thrust surfaces in the outer shroud of the nozzle. The outer shroud is mounted to the engine housing through a spline having the necessary radial freedom to allow the relatively hot outer shroud to expand outward relative to the cooler engine case. The outer shroud is sealed to the engine case by a metal piston-ring seal. Pressure forces acting in a downstream direction on the inner shroud of the nozzle are transmitted to the outer shroud through a series of pheripheral air foil shaped struts which cross the nozzle inlet duct. The clearance between the vanes and the inner shroud must be made sufficiently large so that it does not close completely and prevent rotation of the vanes when the engine is first started and the vanes reach their operating temperature much more rapidly than do the shrouds. This creates a disadvantage in that a large portion of this clearance reoccurs when the shrouds reach their final generally vane equal operating temperature. This excess clearance causes an undesirable reduction in the aerodynamic efficiency of the turbine stage.
The abovementioned vane clearance problem can be solved by using a nozzle having an additional shaft and thrust surface added to the inner end of the vanes co-linear with those on the outer end thereof. The inner shroud of such a nozzle is made of a series, one per vane, of circumferential segments. The clearance between the vanes and the spherical outer surface of the segments is maintained by close dimensional control of thrust surfaces on the vane, the shroud segment, and a retaining collar for the shroud segment. In such an apparatus, the shroud segment moves radially with thermal expansion of the vanes while maintaining a constant clearance with the edge of the vanes. Circumferential movement of the inner and outer shrouds relative to each other is provided for by stepped edges on the segments which overlap each other. As the ring of inner shroud segments changes its diameter sealing with the inner shroud is effected by a pair of metal piston-ring seals. This design uses the vanes themselves to support the inner shroud and to transmit the downstream pressure load acting upon the inner shroud to the outer shroud. This eliminates the struts in the inlet annulus but causes a substantial increase in actuation force required due to the additional bushing loads imposed by the inner shroud pressure load. A further increase in friction and therefore actuation force can be caused by misalignment of inner shroud vane bores with those in the outer shroud as a result of manufacturing inaccuracy.
The present invention provides a variable geometry turbine nozzle which has none of the disadvantages of the above-mentioned prior art variable geometry turbine nozzles.